The present invention relates to a scalable audio coding system in which an audio signal is coded as a plurality of independent layers.
"Audio coding" refers generally to the art of representing audio signals in an efficient manner. Typically, an input audio signal (analog or digital) is coded as a digital signal that occupies less bandwidth than the original signal. An encoding system codes the original audio signal into coded audio data. Sometime later, a decoding system decodes the coded audio data and generates a reconstructed audio signal therefrom.
A variety of audio coders are known in the art. Each may possess relative efficiencies over others in certain coding contexts. Some audio coding systems, for example, are quite simple in implementation and require little processing power by either an encoding system or a decoding system. However, the simple coding systems may not code audio data signals very efficiently. Other, more powerful coding systems may code audio data signals efficiently but may be very complex in implementation. The complicated coding systems may require encoding systems and decoding systems to be very powerful. Often, the design of an audio coding system is impacted directly by the types of audio signals that are to be coded, the bandwidth available for transmission of coded audio data and the processing power of either the encoding system or the decoding system.
Increasingly, particularly in multi-media applications for wide area networks, it is not possible to determine the types of audio signals that will be coded, the bandwidth available for coded audio data or the processing power of decoding systems. In fact, coded audio data may be delivered over channels having variable bandwidth to decoding systems having variable processing power. To code audio signals in a manner that uses the resources of a powerful decoding system effectively, an encoding system may have to encode the audio signal according to a first coding scheme. However, to code an audio signal in a manner that does not overwhelm the resources of a less powerful decoding system, an encoding system may have to code the audio signal according to a second, more rudimentary audio coding scheme. Such repetitive encoding of a single audio signal leads to inefficient use of the encoding system. Accordingly, there is a need in the art for an audio coding system that provides for flexible coding of audio signals. Such a coding system should encode audio signals in a manner that permits rudimentary decoding systems to reconstruct an audio signal from the coded audio data. However, the audio coding system should also represent the audio signal in a manner that effectively uses the resources of a more powerful decoding system. Further, the audio coding system should permit an encoding system to code audio signals only once in such a manner that it is applicable for use with both rudimentary and powerful decoding systems.